Determination of electron abundances in interstellar clouds

A new method for estimating electron fractions in shielded molecular clouds is proposed on the basis of gas phase ion-molecule reactions which involves measuring the quantity Z=n (H2) x [n (HCO+)/n (CO)]. Applied to existing data, it yields upper limits to x/sub e/ in the range from 10-8 to 10-7 for a variety of clouds, warm as well as cool. An upper bound to the cosmic ray ionization rate is also obtained